272 Prof. R. W. Wood on tlw Anomalous Magnetic 
employed in some of the work, though there does not appear 
to be any especial benefit derived from its nse. 
As is well known, when the nicols are set for extinction, 
and the light is restored in the spectroscope by the excitation 
of the magnet, a dark band enters the red end of the spectrum 
and moves towards the violet, when the analysing nicol is 
turned in the direction in which the rotation of the plane of 
polarization has occurred. This means of course that the 
rotation increases as the wave-length decreases. In the case 
of the neodymium film, the reverse was found to be true, the 
band moving up from the blue towards the yellow, until it 
was driven into the absorption-band and lost, indicating that 
the rotation is abnormally great on the short wave-length 
side of the band, as is the case with the vapour of sodium. 
No very marked effect could be detected on the red side. 
We will now examine in detail some of the effects which 
are seen with films of varying thickness. 
The phenomena which are to be discussed occur at the 
absorption-band at wave-length 5790, and as the appearance 
of the band is a little different from that exhibited by solu- 
tions of the salt it seemed best to photograph it. A film was 
pressed between two plates of glass in the form of a very 
acute prism, which when placed in front of the spectrograph 
slit, and illuminated with white light, gave a photograph 
which showed the actual form of the absorption curve. The 
absorption-band is seen to be double, a strong one with its 
centre at 5790, and a fainter one close to it and on the blue 
side (fig. 1). A barium-iron arc was impressed on the same 
plate as a comparison spectrum. 
When a very thin film is placed between the poles of the 
magnet and the current turned on, the restoration of the 
light is only noticeable in the immediate vicinity of the ab- 
sorption-band. A bright and rather narrow band (about 
12 A.E. in width) appears exactly at the centre of the 
absorption-band, that is at wave-length 5790. On the blue 
side of this there appears a fainter and broader band midway 
between the two absorption-bands, while still further along 
there is another narrow band, in coincidence with the fainter 
of the two absorption-bands. 
A photograph of this " magnetic spectrum " was made, 
but it is on too small a scale to reproduce well, and I have 
accordingly made an accurate drawing from it on the same 
scale as the absorption spectrum. The state of polarization 
in this magnetic spectrum appears to be most peculiar. If 
the analysing nicol is turned a degree or two in the direction 
in which the rotation has occurred, the bright band at wave- 
